The present invention relates to an apparatus for data transmission which comprises an inquiry unit in the form of an emitter-receiver and a transponder arranged as a receiver-emitter, the transmission of data taking place at least in one direction by means of a phase modulated emitter, and a phase modulator and wherein the phase demodulator comprises an input for the phase modulated signal and for an unmodulated signal of reference. Such installations serve more specially for so called identification systems wherein the inquiry unit excites the transponder to deliver and transmit a coded answer and wherein the answering code is evaluated in the inquiry unit in order to determine the identity of the answering unit, or of an object associated with the answering unit or the authenticity or authorization of this object or a person. Between the inquiry unit and the transponder there usually exists a wireless transmission, whereby high frequency coils are provided as antennas, said coils serving at the same time as emitting and receiving antennas. Preferably, the transmission of data takes place by means of a phase modulated high frequency signal in order to achieve to the extent possible an undisturbed transmission.
The GB-A-2,164,825 described an identification system of the above mentioned sort. The emitting frequency of the inquiry unit is divided by two in the transponder and the answer is transmitted with half the emitting frequency to the inquiry unit. The emitting frequency is also divided by two in the inquiry unit and this half emitting frequency id delivered to a phase demodulator as a frequency of reference. Under these circumstances, only insignificant problems exist with respect to the position of the phase of this frequency of reference and of the answering signal, because the position of the phase of the unmodulated signals amounts either to 0.degree. or 180.degree.. However, it is desired to select a great distance between both transmission frequencies, that is that these frequencies should differ from each other by a factor of about 4 to 10. Under these circumstances there does not exist a clearly defined position of the phase between the signal of reference and the answering signal which gives rise to particular problems. Moreover, the known system works with two coils each of which however enables operation with a dividing ratio of 2 but hinders a miniaturization.